Forced air heating, ventilating and air conditioning (HVAC) units perform cooling functions by using a refrigeration cycle to remove heat from air that is circulated over evaporator coils, and heating functions by using a furnace, heat pump, or other heating element to heat air from a source vent and blowing the heated air through return vents. Air contains two kinds of heat, sensible heat, which is attributed to the temperature of air molecules, and latent heat, which is attributed to the heat of vaporization of the water vapor contained in air. By measuring the temperature and humidity at the source and return vents of a HVAC unit, the amount of heat added or removed by the HVAC unit may be calculated to determine the system load due to removing sensible versus latent heat.